Propellants have numerous and widespread applications including in vehicles with air bag modules. Among other parameters, propellants can be characterized based on their stability when burning in an air bag inflator. Preferred stability relates to a constant and/or uniform burning of the propellant, along its length, from its outer surface inwardly to its center. A quantitative representation related to propellant combustion stability is in the form of a burn rate exponent n, which is a material parameter defined by [ln(R.sub.2 /R.sub.1)]/[ln(P.sub.2 /P.sub.1)], where R.sub.2 is burn rate at pressure P.sub.2 and R.sub.1 is a burn rate at pressure P.sub.1, (not equal to P.sub.2). The burn rate exponent of most propellants has a value between zero and one. The closer the burn rate exponent value is to one the greater is the instability of the inflator. The operating pressure of the inflator is related to the propellant and inflator hardware properties through the relation: ##EQU1##
P=instantaneous operating pressure of the inflator. PA1 C=constant PA1 A.sub.b =instantaneous burning surface area of the propellant. PA1 A.sub.c =outlet orifice area of the inflator.
It is well known that it is difficult to achieve adequate stability of inflators using propellants with burn rate exponents above about 0.7. Conversely, desired burning stability of the inflator is present when the burn rate exponent of the propellant is no greater than about 0.7.
Certain materials are known to have low and desirable burn rate exponents. Ammonium nitrate based propellants typically have a burn rate exponent of about 0.5. Consequently, ammonium nitrate has a desirable high stability during its combustion. However, ammonium nitrate suffers from certain drawbacks as part of a propellant composition. Especially in comparison with other useful propellant compositions, ammonium nitrate propellants are typically difficult to ignite and have a low burning rate once ignited. It is known to combine ammonium nitrate with another material, such as a secondary explosive, to enhance or achieve the necessary burning rate and/or ease of ignition. Such known propellant compositions have also increased the burn rate exponent of the resulting propellant composition to a burn rate exponent value of greater than 0.7.
It would be beneficial to provide a propellant composition, for use in vehicle inflators or other applications, that includes ammonium nitrate so that the propellant is stable during its burning, while achieving a necessary or desirable propellent burning rate and ignition thereof.